Commercial technologies developed for vehicular “runflat” or “airless” tires include Michelin's Tweels and Resilient Technologies' “non-pneumatic tire.” Both of these use a honeycomb configuration for consumer applications. Runflat technology is also important to military applications. A representative military vehicle may impose 10,000 lbs of vehicle load per tire. To enable the vehicle to operate after tires are perforated by terrain or gunfire damage, passive runflat systems are employed inside the tire. The runflats currently found on the military vehicle are designed to provide mobility for a short time after a tire goes flat, but weigh approximately 100 lbs per tire. In addition to runflat technology, the vehicle utilizes a central tire inflation system (CTIS) to operate effectively across different terrain conditions.
Poisson's ratio (v), named after Simeon Poisson, is the ratio of the relative contraction strain, or transverse strain (normal to the applied load), divided by the relative extension strain, or axial strain (in the direction of the applied load). Some materials, called auxetic materials, have a negative Poisson's ratio (NPR). If such materials are stretched (or compressed) in one direction, they become thicker (or thinner) in perpendicular directions.
NPR materials have attracted significant interest due to their unique behaviors. Unlike conventional materials, a NPR material may shrink when compressed along a perpendicular direction. One result of this behavior is that the material can concentrate itself under the compressive load to better resist the load. Thus, a NPR material becomes stiffer and stronger as the amplitude of the load increases. It has also been found that NPR can improve material/structural properties, including enhanced thermal/shock resistance, fracture toughness, indentation resistance and shear modulus. [1-3].
Auxetic and NPR structures have been used in a variety of applications. According to U.S. Pat. No. 7,160,621, an automotive energy absorber comprises a plurality of auxetic structures wherein the auxetic structures are of size greater than about 1 mm. The article also comprises at least one cell boundary that is structurally coupled to the auxetic structures. The cell boundary is configured to resist a deformation of the auxetic structures.
The vast majority of auxetic structures are polymer foams. U.S. Pat. No. 4,668,557, for example, discloses an open cell foam structure that has a negative Poisson's ratio. The structure can be created by triaxially compressing a conventional open-cell foam material and heating the compressed structure beyond the softening point to produce a permanent deformation in the structure of the material. The structure thus produced has cells whose ribs protrude into the cell resulting in unique properties for materials of this type.